Flood Hazard Mitigation by Actions in the Hillslopes: Does the Context Change the Assessment

Flood hazard mitigation by actions in the hillslopes: does the context change the assessment of efficiency ?

Saâdia Benmamar 1, Christine Poulard2, Pascal Breil2, Etienne Leblois2, Jean-Baptiste Faure2, André Paquier2

1École Nationale Polytechnique, Laboratoire de Recherches Sciences de l’Eau, 10 avenue H. Badi BP 182 Harrach –Algiers, Algeria

2Irstea, Hydrology-Hydraulics Research Unit, 5 rue de la Doua CS 70077, 69626 Villeurbanne Cedex, France

For sustainable and integrated flood management, small actions in the hillslopes and non-structural measures appear interesting, either to diminish the need for large flood mitigation infrastructures (whether sewerage networks or hydraulic structures in the river – which may have severe impact on the river ecosystems), or as complementary to these structures. However, the effect on flood mitigation of land-use modification and small storage or runoff control facilities is still debated in scientific literature. The effect of various structures spread over the catchment is difficult to assess, and hazardous to generalize from one studied catchment to another, which explains why the debate is still open.

This study contributes to identify context features that could also explain constrasting results. Focusing on a West-Mediterranean Northern and Southern countries literature, we compare first traditionnal and modern hillslope actions against runoff in both countries. Then, we search in the physical contexts differences that might explain why actions in the hillslopes are more studied in Europe than in Maghreb. But the priorities of national or regional policies also explain differences in the perception of efficiency: the interest of hillslope actions is different if the aim is to limit erosion and pollutant transfer and/or to mitigate large floods. Pollution and how ecological status is taken into account in flood mitigation project assessment are also crucial points.

However, the effect on flood mitigation of land-use modification and small storage or runoff control facilities is still debated in scientific literature. The effect of various structures spread over the catchment is difficult to assess, and hazardous to generalize from one studied catchment to another, which explains why the debate is still open.

Flood management is a field of investigation with scientific difficulties (hazard characterization and flow dynamics) but also human, economic and political issues (how to reconcile the natural hazard sand development, on which projects funding prioritization criteria flood prevention). The focus is often on urban and suburban areas, because most of the issues are concentrated, and these places are constantly changing.

· The density and the size of the cities continue to grow while the hydraulic capacity of the sewerage networks deteriorates.

·Waterproofing of soil due to the facilities (buildings, roads, car parks...) limit rainfall infiltration into the soil and accentuate runoff. This often causes the saturation and discharge of stormwater sewerage systems. The result is more or less important and often rapid flow in the streets.

It must therefore be distinguished between:

· flooding caused by rivers overflow,

· what is truly the realm of urban runoff and suburban runoff.

1 Principle of development on flood risk management

The principles of possible actions flood risk management matters can be grouped into two broad categories: structural or non-structural measures.

Structural measures (flood routing, improved transit flood flows and the protection of stakes) aim to achieve improvements in networks and the watershed to reduce overflow or protect areas at stake. They act on the natural hazard to the right of the inhabited places in the current state of the watershed. Non-structural measures are additional actions that will help:

• manage the residual risk, since the structural measures do not eliminate the risk beyond the flood design of the structures and risks are assessed based on the occurrence of floods in Centennial minimum or historical flood if upper,

• to reduce the existing vulnerability by area remains flooded, thus addressing the existing issues (shares on the frame, warning and informing people, crisis management) to prevent any increased hazard and challenges in the future through a regulation.

Optimization and selection of the desired pattern is to find the best compromise in terms of impacts and stresses present, acting on:

· the reduction of inputs upstream of the area concerned,

· improving the flow capacity of the networks to the law of conservation areas (urban crossings), and localized protections,

· control of runoff in the watershed.

2 Problems in Algerian cities

Algeria is one of the southern Mediterranean countries which suffered from devastating floods. While in the past these floods were especially known following the overflow of major rivers in the large agricultural plains (Chellif, Soummam, Sebaou ...) since thirty years, they have mainly affected major cities and urban centers, they have produced many casualties and considerable material damages (Bab El Oued floods in 2001 and 2013; Ghardaia, 2008; Annaba in September 2009 and February 2015, El Taraf, 2012, Algiers, 2015 ...) [2, 5].

Photo 1 : Flood of Bab El Oued 2001

In current situation, Algeria is very exposed to the flood risk, whether by river overflow or sewerage network outflow

2.1 Recurring problems on the rivers

Whether in small or large rivers, flow is hampered by reductions in flow sections due to uncontrolled, embankments and illegal occupations of the bed and the banks, and to many linear infrastructures (roads, railways,...) (eg. Oued Reghaia (photo 1)).

Photo 2:Oued Reghaia

2.2Recurring problems on sewerage network

The sewerage networks have generally been designed for a return period of 2 years in previous masterplans. Low concentration time and the steep slopes of some watersheds where urbanization is highly developed without preserving the lines of flow can be originally flooding in a fast and violent way, that can cause disasters. Overflows are generated by intense showers of short duration; they are aggravated by conditions of saturation of soils penalty following a succession of rainy episodes leading to strong overlapping of rain, and that can cause mudslides. In low slope areas, where drainage runoff is using pumping stations, inadequate pumping capacity also leads to frequent overflows.

3 Current techniques in the management of stormwater in Algerian cities

The Algerian authorities, overwhelmed by the magnitude of the problem of urban water and seeking solutions, proceeded with the creation of urban water management agencies for the most important cities of the country, with mixed partnership contracts. Solutions presented as innovations appear rather as old-fashioned, classical methods: rainwater is merely expected to be managed through gutters or pipes towards one or more outlets, and released into the environment after temporary storage and treatment in a sewage plant. The sustainability and actual cost of these proposals should be assessed more carefully.

3.1 Sewerage networks

When a sewerage system is defective, the classic solution adopted is repair or restructure by combining the following [4]:

• Strengthen, Renew or double the sections of the existing network: eg. duplication collector OuedM’Kacel at Bab el oued carried following the flooding of 2001. It will allow to drain storm water to the sea, while wastewater will junction to a collector who will be deviated to the East of the capital, Baraki or Reghaia step.

• Building of new culverts (e.g. gutters stairs installed throughout Algerian territory).

• Equipping thunderstorm weir networks that provides storm basin: they allow directly reject amounts that exceed the capacity of the network in the natural environment. (eg. Type storm sewer works to vortex on the collector “ouedM’Kacel”)

3.2 Rivers

Developments on the rivers are:

· Development of dikes sidewalls and the growing upstream urban areas;

· Recalibration of the watercourse.

The level of protection of the facilities would decrease: 50 instead of 100th, 10 years instead of 20 years and 5 years instead of 10 years

These methods of urban drainage recommended, contribute towards the surrounding streams, in oversized and expensive concrete pipes, storm water, quickly simply displace the problem of floods from one locality to another, without truly solving the problem of a final and lasting. If these methods that no longer have courses in developed countries because costly and unsustainable, why are they preferred the most appropriate methods, greener, and cheaper in Algeria?

4Sustainable urban drainage systems

Used in urban stormwater management techniques have a majority was inspired by ancestral techniques in arid areas around the world (eg: ancestral hydraulic system of Ghardaia).

In recent decades many countries have adopted alternative methods stormwater management.

The issue of Sustainable urban drainage systems is to support existing networks by storing and controlling stormwater, so firstly, to participate in the management of flood risk, and secondly, reduce pollution natural environments. They differ from the traditional networks by their ability to integrate into the urban fabric, in the form of valleys, ditches, trenches, soakaways, basin, green roofs [1, 6].

Comparing cost and benefits associated with the great floods: conventional methods may be preferable and have the advantage of being based on classic design methods which have proven dedicated tools.

By cons, to comprehensively evaluate the costs, ithe following points should also be taken into account:

• The damage reduction for small floods, even local, individually inexpensive but accounting for much in the expected annual damages because of their frequent occurence;

• The ecological consequences of overflows on the natural river functions, regarding the local and national environmental policies (see for instance the Water Framework Directive of the European Union, imposing to preserve and improve the ecological status of rivers).

Many obstacles remain:

• Cost and feasibility of Sustainable urban drainage systems are case-dependent. A specific detailed study is necessary, for each project. Discussions may arise about the actual costs for the public project-manager. Muti-objective projects are also more difficult to assess.

• A real lack of information regarding these techniques and potentialities among stakeholders likely to propose, prescribe them or use them.

• Need of expertise for Sustainable urban drainage systems : the methodology and tools are less straighforward than for pipes design ; it is still in part a research field. Working methods must be adapted, training must be available. Feed-back must also be shared.

• Not all the stakeholders have interests in these techniques (traditional consultants, private companies or persons who do not want to support the cost of rainfall management on their plot and prefer collective solutions….)

• Difficulty of design and management because of the diversity of structures and their small size .

• Difficulties of maintenance of rainwater management structures sharing other functions (grassed swales to be tended as public gardens, water retention on sports or recreational facilities, porous roadsides and pavements where rainwater infiltrates ...)

• Fear of novelty., and lack of acceptance of experimental designs (in partnership with universities or not), with no full guarantee of success.

Conclusion

Integrated flood management offers many opportunities for the development of water supply, both locally and regionally. They require more studies, including innovative developments, and careful maintenance. The issue is also to assess their effects in a comprehensive way: they do not only concern large floods, but may also help manage small local problems, and can bring multi-objective solutions, including public awareness (with visible facilities) and resource management (with storages and infiltrations). This may contribute to necessary adaptations to drought, that Algeria suffered for many decades and risk likely to worsen in the event of a global climate change.

References

1 Azzout Y., Barraud S., Cres F.N., Alfakih E., (1994) Les techniques alternatives en assainissement pluvial, Ed. Tech et Doc, Lavoisier, 372 pages, 1994.

2 Bahlouli, L., (2009). Crues et inondations en Algérie. Actes de l’atelier scientifique et technique sur « L’Outil spatial au service du développement», pp. 17-25, Alger 28-29 mars.

3 Chocat B., (1997). Encyclopédie de l’Hydrologie Urbaine et de l’Assainissement, Edition Lavoisier Tech & Doc.

4 Le Jallé, C. (2013) «La gestion des eaux pluviales (GEP) en milieu urbain dans les pays en développement». Ps-Eau, 38pages

5 Menad W., (2012) «Risques de crue et de ruissellement superficiel en métropole méditerranéenne : cas de la partie ouest du Grand Alger». Thèse De Doctorat en Géographie de l’universitéParis Diderot - Paris 7.

6 Poulard-Semra C., Berthie E., Breil P., Labbas M., Henine H., Hauchard E., Radzicki K., (2013). Managing rainwater all the way down: what urban and rural practitioners can teach one another and do together. Novatech